Ensure ISO-8859-1 when sending to gateway

ISO-8859-1 is expected for the HTTP API docs, so I'm assuming the same holds for the XML API.

For the XML API to accept unicode characters in messages the OP element must be set to 9, but the gem doesn't set this element when posting.

Noticed similar changes being made to the python library a while ago: https://github.com/PSWinCom/pswinpy/commit/6d1e6dd28f50acc5b4a36b6bc73dcb4bebd0374f